In a general feeding structure of a conventional small-sized antenna used in wireless communications, a coaxial line is directly brought into contact with an antenna to perform feeding. For monopole antennas, a + part of a coaxial line is brought into contact with an antenna to perform feeding. For dipole antennas, + and − parts of a coaxial line are brought into contact with an antenna to perform feeding.
These methods cause an unbalance condition between feeding lines of an antenna, so impedance matching becomes quite difficult. Additionally, a contact point between the antenna and the feeding line frequently varies, so the characteristics of the antenna are not constant, thus reducing the efficiency of the antenna.
As depicted in FIG. 1, U.S. Pat. No. 4,772,895 circular plateloses an antenna 500 that broadens the frequency response. The antenna 500 includes a feed port 550 having a signal feed portion and a ground portion, a first helical antenna element 520 having opposed ends and exhibiting a first pitch and a second electrical length, one end of the first helical antenna element 520 being coupled to the signal feed portion of the feed port, and a second helical antenna element 540 having opposed ends and exhibiting a second pitch and a second electrical length.
The second helical antenna element 540 is coaxially wound around a portion of the first helical antenna element 520, one end of the second helical antenna element 540 is coupled to the ground portion of the feed port 550, and the second pitch is equal to approximately ½ of the first pitch and the second electrical length is equal to approximately ⅓ of the first electrical length.
The antenna 500 is provided with a cylindrical spacer means 530 that is coaxially situated between the first and second helical antenna elements 520 and 540 to electrically insulate the first and second helical antenna elements 520 and 540. The spacer means 530 is sufficiently thin such that the first helical antenna element 520 is tightly coupled to the second helical antenna element 540 so as to broaden the frequency response exhibited by the first helical antenna element 520.
In the conventional antenna, the spacer means is situated between the first and second helical antenna elements, and is used to ground the antenna elements. The conventional antenna is problematic in that it cannot overcome the unbalance condition that is a problem in the conventional antenna, thus causing low efficiency, and it is difficult to miniaturize.
With respect to the unbalance condition, helical antennas are chiefly classified into normal mode antennas and axial mode antennas. The case where the circularly shaped circumference of the helical antenna is considerably smaller than a wavelength corresponding to a working frequency falls under normal mode. Generally, helical antennas used in wireless communications devices have normal mode.
The characteristics of the normal mode helical antenna are that the characteristic impedance is considerably large and the radiation resistance value corresponding to actual radiation power is small. Accordingly, the input impedance value is considerably large in total and considerably different from the output impedance, 50 Ω, so the reflection loss is increased. This is the inherent unbalance condition of the conventional helical antenna that is used as a general wireless communications receiving antenna.
As illustrated in FIG. 2, U.S. Pat. No. 5,661,495 circular plateloses an antenna device 200 having circuits 230 for transmitting and/or receiving radio signals as well as a chassis 250 and a feeding point providing the electrical coupling of the antenna device to the communication equipment, which includes a hollow helical antenna 210 fixed externally on the chassis 250 and an antenna rod 220 slidable through the helical antenna 210, the helical antenna being coupled constantly via the feeding point to the circuits 230.
Meanwhile, the bandwidth of the helical antenna 210 is increased, a tuned ground surface is arranged near the feeding point, a direct Galvani electrical contact is not formed, and the ground surface is coupled to the protective earth of a communications device and can catch mirror current.
In the conventional antenna device, when the antenna rod is extended from a housing, the antenna rod and the helical antenna are coupled in parallel to the circuits 230. When the antenna rod 220 is retracted into the chassis 250, only the helical antenna is coupled to the circuits 230.
Meanwhile, a circuit equivalent to the case where a helical antenna is installed in a general cylindrical structure chiefly consists of a feeding part and the parallel resonance parts of L and C. This conventional helical antenna reduces the length of the conventional monopole antenna but has the same resonant frequency as the conventional monopole antenna. In this case, the Q value is increased due to the parallel resonance of L and C, so a band of frequencies is narrowed.
Accordingly, with reference to a graph showing the electrical characteristics of a conventional helical antenna in Voltage Standing Wave Ratios (VSWRs) and a Smith Chart showing impedance measurement data, as shown in FIGS. 4a and 4b, as the VSWR value is increased and the impedance value is away from the center of the Smith Chart, the reflection loss of the antenna is increased and the bandwidth of the antenna is narrowed.
The bandwidths of the conventional antennas having structures shown in FIGS. 1 and 2 are each defined as a band of frequencies having a VSWR value equal to or less than 2. Accordingly, the conventional antennas each have a VSWR value ranging from 5 to 18 and the impedance value of the Smith Chart is considerably away from a value of 50 Ω situated at the center of the Smith Chart, so it can be appreciated that the reflection loss value of the antenna increases and, therefore, the conventional antennas each have a relatively narrow band of frequencies.
Additionally, the conventional antenna is problematic in that the efficiency of the conventional antenna is deteriorated because the unbalance condition that is a problem in the conventional antenna is not overcome.